The present invention relates in general to microwave filters and in particular to a band pass microwave waveguide filter that preserves the polarization of the input signal.
Microwave and millimeter wave continuum imaging systems are used in contraband detection, material characterization, remote sensing and astronomical applications. During the measurements, the polarization of the input signal must be preserved in order to extract the desired signatures without measurement bias. Although a conventional filter can be designed to meet the out-of-band requirements, the conventional filter does not preserve the polarization of each mode. On the other hand, conventional filters that preserve dual polarization exhibit poor out-of-band response.
In traditional system configurations used to achieve this functionality, an orthomode transducer (OMT) separates the polarization into vertical and horizontal polarization states and allows for the use of rectangular waveguide structures cascaded with waffle filters to define the frequency band of the two polarization states. However, this configuration is complex, requires two high performance filters and an OMT. In addition, the structure does not have much control in the shift of higher order modes and is not able to suppress the repetition of the fundamental mode by increasing the filter order. Therefore, there is a need for a microwave filter that preserves polarization of the input signal and has excellent out-of-band rejection achieved by suppressing spurious modes.